"He may not be someone who's going to come into the United States to conduct the attack, but what makes him more dangerous is he's out there plotting the attacks and recruiting people to actively do that," LeBlanc says.

The breakthrough came when LeBlanc, an FBI counter-terrorism agent, linked Shukrijumah to the thwarted New York subway suicide mission last fall -- the biggest post-9/11 terror investigation.

Najibullah Zazi and Zarein Ahmedzay admitted they planned to blow themselves up using homemade bombs.

Prosecutors say it was Shukrijumah who called the shots -- probably from somewhere along the Afghan-Pakistan border.

"[Shukrijumah] was the one that convinced ... them to come back to the United States and conduct the attack here," LeBlanc says.

But Shukrijumah's mother -- who did not wish to be named in order to protect her privacy -- says authorities are using her son as a scapegoat.

"That is not my son. My son is not a violent person. He is very kind, generous," she says.

Shukrijumah -- the eldest son of a Saudi Imam -- came to America as a young child.

They settled in Brooklyn, New York, where his father preached at a mosque. They lived at a nearby house before moving to Florida in the mid-'90s.

His father, who is now dead, opened a small mosque near Fort Lauderdale.

Meanwhile, Shukrijumah worked at odd jobs, including selling used cars. His family says it's how he paid for courses, including computers and chemistry, at a small college in South Florida. He even took classes to improve his English.

A few years later, when the FBI said they wanted to find him, Shukrijumah's English professor remembered videotaping him in class and turned over the tape to the FBI. That proved crucial six years later.

On a hunch, LeBlanc asked agents in New York to show that video of Shukrijumah to would-be bomber Zazi.

"From that video, he was able to make an identification," LeBlanc says.

The FBI says it now has a more detailed profile of Shukrijumah in part, from 9/11 mastermind Khalid Sheik Mohammed.

Just before 9/11, the FBI says Shukrijumah crossed the U.S. by train. Later, he scoped out the Panama Canal as a target.

He went to Trinidad, London and -- by June 2001 -- Afghanistan.

On 9/11, his mother says he called home for the last time.

"He called me and he said, 'Did you hear what happened with so and so and so?' He said, 'They're putting it on the Muslims. I said, 'yes.'"

She says she told him not to come home.

"And he was arguing with me. He said, 'No, I didn't do nothing. I will come, don't worry about this,'" she recalls.

After that, she says, she never heard from him again.

Shukrijumah's mother adamantly denies her son is directing al Qaeda attacks. But when asked about Faisal Shahzad -- who has pleaded guilty to federal terrorism charges in the attempted car bomb attack in Times Square in May -- she says that sometimes "you have to do something very alarming for the people to wake up."

"It's not because you hate them or you want to destroy them or you want to hurt them," she says.

When asked if she has a message to send her son, she says she has nothing to say.

"He [has] his own guide in his own heart," she says.

For the FBI, it's all about staying one step ahead. Where will al Qaeda -- and their alleged chief of operations -- strike next?

"He's definitely focused on attacking the United States and other western countries," LeBlanc says.

The FBI believes Shukrijumah is likely in Pakistan's lawless tribal region of Waziristan.

Shukrijumah moved up the ladder after two others who directed outside operations for al Qaeda were killed in suspected U.S. drone attacks, the FBI says.